1. Field of the Invention
The present invention relates to a radiographic image processing device and method which performs image processing related to a scattered radiation removal grid for a radiographic image and a program which causes a computer to perform the radiographic image processing method.
2. Description of the Related Art
In the related art, when a radiographic image of a subject is captured using radiation that is transmitted through the subject, the radiation is scattered in the subject and the scattered radiation (hereinafter, also referred to a scattered ray) causes a reduction in the contrast of the acquired radiographic image. For this reason, in some cases, when a radiographic image is captured, a scattered radiation removal grid (hereinafter, simply referred to as a grid) is provided between a subject and a radiation detector which detects radiation and acquires a radiographic image such that the scattered radiation is not emitted to the radiation detector. When imaging is performed using the grid, radiation which is scattered by the subject is less likely to be emitted to the radiation detector. Therefore, the use of the grid makes it possible to improve the contrast of the radiographic image.
In contrast, when imaging is performed using the grid, a subject image and a stripe pattern (grid strips) corresponding to the grid are included in the radiographic image, which makes it difficult to see the image. For this reason, a process is known which removes grid stripes from a radiographic image (see JP2012-203504A). In addition, the grid stripes are generated in a case in which a stationary grid is used for imaging in a stationary state and is not generated from a radiographic image in a case in which an oscillating grid (Bucky grid) oscillates and is then used. Therefore, it is possible to acquire a high-quality radiographic image without a grid stripe, without performing the grid stripe removal process.
The grid has a structure in which radiopaque lead and a radiolucent interspace material, such as aluminum or fiber, are alternately arranged with a fine grid density of, for example, about 4.0 lines/mm. Therefore, the grid is weighty. For this reason, in portable radiography which is performed in, for example, a hospital room, the grid needs to be provided between a lying patent and a radiation detector and the weight of the grid causes an increase in the burden of an arrangement operation on a radiographer and an increase in strain on the patient during imaging. Further, in the case of a convergence-type grid, density unevenness is likely to occur in the radiographic image due to the oblique incidence of radiation.
For this reason, a process has been proposed which captures a radiographic image, without using a grid, and gives an image quality improvement effect, which can be obtained by removing scattered radiation using a grid, to the radiographic image through image processing (see U.S. Pat. No. 8,064,676B and C. Fivez et al., Multi-resolution contrast amplification in digital radiography with compensation for scattered radiation, 1996 IEEE, pp. 339-342). The methods described in U.S. Pat. No. 8,064,676B and C. Fivez et al., “Multi-resolution contrast amplification in digital radiography with compensation for scattered radiation”, 1996 IEEE, pp. 339-342 decompose a radiographic image into a plurality of frequency components, perform a scattered radiation removal process of controlling contrast or latitude for a low-frequency component which is regarded as a scattered radiation component, and combine the processed frequency components to acquire a radiographic image from which the scattered radiation component has been removed. In the method described in U.S. Pat. No. 8,064,676B, the scattered radiation removal process is performed by multiplying a low-frequency component by a gain corresponding to the hierarchy of the low-frequency component and the pixel value of the low-frequency component. Here, the gain is less than 1. The gain has a smaller value in a lower frequency band and is reduced as the pixel value increases. The method described in C. Fivez et al., “Multi-resolution contrast amplification in digital radiography with compensation for scattered radiation”, 1996 IEEE, pp. 339-342 uses a table for converting a low-frequency component according to the pixel value thereof. In the method, lower frequency bands are increasingly reduced in a geometric progression manner.
According to the methods described in U.S. Pat. No. 8,064,676B and C. Fivez et al., “Multi-resolution contrast amplification in digital radiography with compensation for scattered radiation”, 1996 IEEE, pp. 339-342, since no grid is required during imaging, it is possible to reduce strain on a patient during imaging and to prevent the deterioration of image quality due to density unevenness and a grid stripe.
In contrast, a general radiography system acquires three types of radiographic image, that is, a radiographic image which is captured using a stationary grid, a radiographic image which is captured using a Bucky grid, and a radiographic image which is captured without using a grid. In this situation, in a case in which a grid stripe removal process is performed for the acquired radiographic images, the grid stripe removal process is also performed for the radiographic image which is captured without using a grid. Inversely, in a case in which a scattered radiation removal process is performed, the scattered radiation removal process is also performed for the radiographic image which is captured using a grid. As such, when a process which is not necessary for the radiographic image is performed for the radiographic image, the quality of the radiographic image deteriorates significantly, which makes it difficult to make a diagnosis with high efficiency.
For this reason, a method has been proposed which detects whether a grid is present and the type of grid, on the basis of, for example, the weight of the grid or projections that are formed on the grid according to the types of microswitch and grid, and performs image processing corresponding to whether a grid is present and the type of grid, in systems that use a grid, do not use a grid, and use a plurality of types of grid (see JP2003-260053A). In particular, JP2003-260053A discloses a method which selects whether to perform a grid stripe removal process on the basis of information indicating whether a grid is present.